Photoionized gaseous nebulae and magnetized stellar winds: The evolution and shaping of HII regions and planetary nebulae

The early evolution of hydrogen(+) (H II) regions is controlled by the prop erties of the star-forming cloud cores. The observed density distributions in some young H II regions indicate that the power-law stratifications can be steeper than r(-2). Ionization fronts can overrun these gradients and th e ionized outflows are strongly accelerated along these steep density distr ibutions. Thus, photoionized regions can either reach pressure equilibrium inside the inner parts of the high-pressure cores [with sizes and densities similar to those observed in ultra compact (UC) H II regions], or create b right H II regions with extended emission. The density inhomogeneities engu lfed within the ionization fronts create corrugations in the front, which i n turn drive instabilities in the ionization-shock (I-S) front. These insta bilities grow on short time scales and lead to the fragmentation of the den se shells generated by the shock fronts. Thus, new clumps are continuously created from the fragmented shell, and the resulting finger-like structures can explain the existence of elephant trunks and cometary-like globules in most H II regions. In the case of planetary nebulae (PNe), wind asymmetrie s and magnetic fields from rotating stars, along with precession of the rot ation axis, can create the wide range of observed PNe morphologies and coll imated outflows (jets). Magnetic collimation and jet formation in PNe becom e very efficient after the flow has passed through the reverse shock of the PN. (C) 2001 American Institute of Physics.